Geotech Report - Initial Findings

Essence of Peregian
215-219 David Low Way, Peregian Beach
Project No. 007-160A,
27 April 2010



       Extreme caution should be exercised if groundwater table lowering is envisaged, as any adjacent structures
       founded in (or above) very loose and loose sands may experience significant (and potentially non-uniform)
       settlement as a result of groundwater level reduction.

       Suitable temporary groundwater control methods would need to be properly assessed, but would most likely
       comprise a system of spear points. As a very preliminary guide, discussions with groundwater dewatering
       contractors with experience in the general area indicate that inflow rates of 0.1l/s to 0.5l/s per square meter
       of excavation base for a depth of 1m to 1.5m below groundwater level and side area (below the groundwater
       level) may be appropriate, but will be subject to the excavation depth and the temporary support method
       adopted. Inflow values will need to be confirmed by detailed analysis and design.

       5.4.2     Permanent Groundwater Control

       In proposed basements, groundwater seepage into the excavation may occur that could adversely affect
       basement wall and floor slab performance and building amenity. Unless a fully tanked basement is adopted,
       control of groundwater seepage by drainage will be necessary. For a ‘non-tanked’ basement, floor slabs will
       need to be cast over a free draining layer (incorporating agricultural drains) graded to sumps so that
       groundwater is removed and pressure does not build-up under the slab.

       Detailed groundwater modelling will be required to confirm the feasibility of adopting drained basements and
       to enable determination of potential inflow values.

       Three options which could be considered (subject to the results of detailed groundwater modelling) for
       permanent groundwater control in the basement are indicated below. Whichever option is selected the
       ‘average’ groundwater level external to the site should not be lowered.

       Option 1:        Fully tanked basement. The basement walls and slab could be designed and tanked for
                        hydrostatic pressures equivalent to the highest predicted groundwater levels at the site. This
                        is the most expensive option.

       Option 2:        Fully drained basement. An appropriately designed drainage layer behind basement walls,
                        and an under slab drainage layer, would be used to control groundwater and remove
                        hydrostatic pressures. It is however recommended that the walls and slabs be designed to
                        accommodate some hydrostatic load if there is a failure / blockage in the drainage system.

                        Any underslab drainage system would need to be properly analysed and designed, however,
                        for initial costing a 200mm thick layer of single sized aggregate should be allowed for,
                        together with 75mm diameter geotextile wrapped agricultural drains laid one way at not more
                        than 6m to 8m centres and connected to sumps.

       Option 3:        Hydrostatic relief system. The basement walls and slab could be designed and tanked for
                        a groundwater level required to balance hydrostatic uplift and the weight of the overall
                        structure (allowing for a minimum factor of safety applied to uplift forces). When
                        groundwater levels rise above this level, a pressure relief system would allow the basement
                        to flood, or alternatively collect the overflow waters within a system of drains and sumps.
                        This option is based on the acceptance of the risk that the basement may flood.

       It is strongly recommended that groundwater inflow volumes are monitored during construction to provide an
       assessment of the permanent inflow rates and that the results of this monitoring be compared with predicted
       groundwater inflow volumes.


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